1. Field of the Invention
The present invention relates to a method of making preforms for optical waveguides, more particularly to a method for controlling the amount of fluorine doping at various locations across the profile of such preforms.
2. Technical Background
Optical waveguides such as optical fiber are typically made using materials of different refractive indices for the core and clad regions. The core region, wherein the majority of light is transmitted, consists of a region of higher refractive index than the clad region. In silica based optical fibers, this refractive index difference is obtained by adding various dopants. Fluorine is a commonly employed dopant for lowering the refractive index, and germania is a commonly employed dopant for raising the refractive index.
To fluorine dope a silica soot preform, such as can be made from an OVD, MCVD, VAD or other vapor deposition process, a fluorine containing gas such as SiF.sub.4 or CF.sub.4 can be flowed over the surface of the soot preform. The fluorine containing gas then diffuses into the soot preform and the fluorine reacts with the soot, thereby doping the silica soot with fluorine. In the past, fluorine incorporated into the soot has been highest near the preform surface which is in contact with the flow of fluorine containing gas.
Methods have been proposed to achieve a more uniform level of fluorine doping or a higher fluorine doping amount on a surface of the soot preform which is opposite the flow of fluorine dopant gas. For example, U.S. Pat. No. 5,210,816 discloses achieving this result by changing the soot density prior to the consolidation step. Basically, this method works on the principle that the lower the soot density, the higher the fluorine dopant level will be. It would be desirable to develop alternative methods which do not require deposition of various layers having different densities.